The present invention is directed to a high temperature diffusion furnace such as that used in the semiconductor industry to heat semiconductor wafers so that, for example, the wafers can be doped with an appropriate material. In particular, the invention pertains to a retention mechanism which maintains a desired configuration of a helical wound resistive wire.
High temperature diffusion furnaces are well known to the semiconductor industry (e.g., see U.S. Pat. Nos. 5,038,019, No. 5,461,214, and No. 6,512,206, the disclosures of which are incorporated herein by reference). Heat treatment in high temperature diffusion furnaces is a part of the manufacturing process of silicon wafers whereby, for example, doping elements such as boron can be introduced into the molecular structure of the semiconductor material. Heating cycles for the furnaces must be controlled accurately with respect to time and temperature. There is also a requirement that the diffusion furnace be made durable enough to withstand repeated heating and cooling cycles. Further, for purposes of the manufacturing processes, it is important that the diffusion furnace quickly reach the desired temperature, maintain the temperature for a preselected period of time and then quickly reduce the temperature to the desired level.
In actual practice, the diffusion furnaces used in the semiconductor industry are substantially cylindrical in shape. All diffusion furnaces are equipped with a process tube in which the silicon wafers are processed. The process chamber is fabricated of quartz, polysilicon, silicon carbide or ceramic and is inserted into the diffusion furnace.
As shown in FIG. 1, a prior art diffusion furnace 20 includes an outer metallic housing 22, usually comprised of stainless steel or aluminum and inner layers 24 of insulating materials such as ceramic fiber. Several helical heating coils 26, 28 and 30 are secured together to form one continuous helical element 29, with the middle heating coil 28 operated at the optimal temperature and the end heating coils 26, 30 operated at a temperature sufficient to overcome thermal losses out of the end of the furnace and to preheat any gases being introduced into the process chamber of the furnace which can comprise one or more process zones. The heating element is generally a helically coiled resistance wire made of a chrome-aluminum-iron alloy. The wire is generally heavy gauge (e.g., 0.289 inches to 0.375 inches in diameter) for longer heating element life at an elevated temperature.
As can be seen in FIG. 1, at either end of the furnace 20 is a vestibule 46, 48. The vestibules 46, 48 are counterbored to accept end blocks 60, 62 which are sized to fit the process chamber 21. The process chamber 21 is suspended between the end blocks 60, 62. The silicon wafers 56 to be heat treated are mounted into boats 54, fabricated of quartz, polysilicon, silicon carbide or ceramic. The boats 54 are then loaded into the process chamber 21 for processing. The boats 54 may be slid manually or automatically into the process chamber 21 or suspended within the process chamber on cantilevered: support arms 59 constructed of silicon carbide or ceramic and quartz.
The maximum permissible operating temperature for the heating element alloy is around 1420xc2x0 C. Since a temperature differential exists between the heating element and the inside of the process chamber, diffusion furnaces are normally operated at a maximum operating process chamber temperature of around 1300xc2x0 C.
A coil-retention mechanism is provided to separate and hold in place the individual coil turns of the helical heating element 29. Maintenance of the correct separation between each coil turn is critical to the operation of the furnace which normally requires a maximum temperature differential of not more than plus or minus xc2xd C., along the entire length of the process zone. Electrical shorting between turns and interference with uniform heat distribution can result if the gaps between the coil turns are varied. The retention mechanism typically comprises rows of ceramic spacers, such as spacers 32 shown in FIG. 2. The rows extend parallel to the longitudinal axis of the helical heating element and are spaced apart around the circumference of the heating element.
Generally the insulation 24 is comprised of a ceramic fiber insulating material having about 50% alumina or more and the balance silica. This insulating material is applied to the exterior of the heating element after the coil turns are positioned within the spacers. The insulation is applied either as a wet or dry blanket wrapped around the heating element or is vacuum formed over the element. After the insulation has dried, sections of the insulation disposed between the rows of spacers cooperate with the spacer rows to keep the coil turns of the helical heating element 29 properly aligned.
As indicated above, the operating temperature of the furnace is generally over 1000xc2x0 C. The furnace typically cycles between temperatures of approximately 800xc2x0 C. when the boats are loaded into the furnace process chamber and over 1000xc2x0 C. during full operation. As indicated above, it is imperative that the furnace quickly reach the operating temperature and quickly cool down after operation.
Failure of these prior furnaces 20 is often due to the inability to control the growth or expansion of the heating element, the inability to prevent failure of the ceramic fiber insulation, the inability of the spacers to properly maintain the spacing of the individual coils of the heating element, and the combined effect of these occurrences, resulting in coil sag.
A problem occurring with insulation involves a tendency for the insulation to shrink with age and temperature changes. As a result, gaps can form between the spacer rows and the insulation sections disposed between the spacer rows as those shrinking sections pull away from the spacer rows in the circumferential direction. The gaps are oriented radially with respect to the coil, thereby forming direct paths through which radiant heat loss can occur.
Another problem results from a lack of sufficient support for the endmost turns of the heating coil, enabling those ends to sag and produce premature failure of the coil.
The present invention relates to an electric furnace which comprises a helical heating element, and rows of spacers for maintaining a spacing between adjacent turns of the heating element, each row of spacers extending from one end of the heating element to an opposite end of the heating element, the rows of spacers being circumferentially spaced apart with reference to a center axis of the heating element, each spacer including circumferentially facing side surfaces defining radial undercuts. Thermal insulation extends around the outside of the heating element. The insulation includes circumferentially spaced portions each disposed within a respective space defined between two adjacent rows of spacers and engaging the radial undercuts of the spacers of the two rows.
Another aspect of the invention relates to an electric furnace having a helical heating element encircled by insulation, and rows of spacers for keeping turns of the heating element spaced apart by predetermined distances. Each spacer includes circumferential side portions facing generally in a circumferential direction of the heating element and converging toward a center region of the spacer to form a radial undercut against which the insulation abuts. The spacers of each row have through holes extending parallel to a longitudinal axis of the hating element. A guide rod extends through the through holes of each row of spacers.
Yet another aspect of the invention relates to a spacer for spacing apart the turns of a helical heating element. The spacer comprises a pair of oppositely facing first side surfaces, and a pair of oppositely facing second side surfaces. One of the first side surfaces includes a pair of projections spaced apart in a first direction. The other of the first side surfaces includes a recess sized to receive the projections. Portions of the second side surfaces converge in a second direction parallel to the first direction to form undercuts.